In "A Poison Tree" by William Blake, what are the prevailing meter and substitition meter, and how are they used in the poem?

The prevailing meter (one that is used most often) in this poem is trochaic (one strong accent followed by one weak or unaccented  syllable) trimeter (three feet to a line) with one leftover accent (masculine ending) at the end of the line:

/   u    /    u    /     u     /

I was angry with my friend:

The substitution meter is iambic (unaccented followed by accented syllable) tetrameter (four feet to a line).

u   /    u    /         u     /        u     /

I told my wrath, my wrath did end.

There are only three lines in the poem that use the substitution meter:

u   /    u    /         u     /        u     /

I told my wrath, my wrath did end

and

u  /    u   /     u   /          u     /

I told it not, my wrath did grow.

and

u     /      u     /         u      /      u     /

My foe outstretch'd beneath the tree

All of the other lines in the poem are in the prevailing trochaic trimeter.

Why does Blake do this? To call attention to those three lines. They are out of "sync" with the others with regard to rhythm (meter). By doing this, I believe the poet forces us to concentrate on these three lines to get his theme across. The poem is about anger and its destructive nature. The lines that don't fit the meter tell us: "I told my wrath, my wrath did end" - if you are angry with someone, talk to him and work it out; "I told it not, my wrath did grow" - if you don't work it out, your anger will grow like a poison tree and; "My foe outstretch'd beneath the tree" - it can "kill" (figuratively speaking).